This invention relates to semiconductive devices. More particularly, this invention relates to such devices carrying circuit elements such as thin film resistors which are trimmed to specified electrical characteristics by the use of a laser beam directed onto the element.
Integrated-circuit components commonly comprise a semiconductor substrate, typically doped Silicon, carrying a combination of active and/or passive circuit elements. In many cases, such circuit elements include thin films of electrically-conductive material forming electrical resistors, and separated from the substrate by dielectric material.
In order to set the value of such a circuit element precisely at a prescribed magnitude, the processing of semiconductive components often includes a procedure referred to as laser trimming. In that procedure, a focused laser beam is directed onto the circuit element, and controlled so as to vaporize or otherwise remove or alter the material of the element. During or following this operation, the value of the circuit element is monitored by associated measuring equipment, and the laser trimming is stopped when that value reaches a directly or indirectly specified magnitude. There have been many disclosures of various means for carrying out laser trimming procedures, e.g. as shown in U.S. Pat. No. 3,699,649, and other patents cited therein.
One of the problems encounted in such laser trimming operations is that semiconductive substrates are not transparent to the laser beam (as are glass substrates), and absorption of laser energy by the substrate can cause substantial generation of heat. This in turn can result in damage to the substrate material, or alteration of the characteristics of regions of the substrate or material on the substrate such as surface dielectric or resistor material, so as to adversely affect the component performance.
In such circumstances, it has been a common practice simply to reduce the power level of the incident laser beam, as by means of filters or the like, to a level sufficiently low that no significant injury will be sustained by the substrate or associated elements. However, that solution to the problem has not been entirely satisfactory since in many cases low-power laser beam are not capable of achieving the required high-performance in trimming the circuit element. For example, at such lower power levels the laser cut generally will not be as clean, and in any event the stability or noise characteristics of the circuit element often will be significantly better when trimmed with relatively high-power laser beams.
Accordingly, it is an object of this invention to provide means and methods for laser-trimming circuit elements on semiconductive substrates at relatively high power levels, yet without generating excessive heat in the substrate.